Mullion systems or mullions provide the central support between a divided window structure. Mullion strength is an important factor for providing wind load resistance to a window. The significance of the mullion is amplified with an increase in the number of adjacent windows and different combinations of windows sought after by builders in order to guarantee a strong window assembly. As the size of the window assembly is increased, the mullion must provide enough reinforcement to ensure the safety of the system.
Mullion systems have traditionally been designed to reinforce adjacent wall panels or windows. However, conventional mullion systems are often difficult to assemble or performance rate as complete window systems. This has limited the number of window combinations that meet new building codes and performance standards (such as AAMA 450 detailed below), especially in places where harsh conditions are a factor, such as coastal regions where hurricanes are prevalent, or in the Midwest, where tornadoes are common.
Additionally, with the increase in frequency and severity of weather events, mullion standards have been raised significantly. One of these raised standards, that has emerged as a leading standard, is the American Architectural Manufacturers Association's (AAMA) 450 Performance Rating Method for Mulled Fenestration Assemblies. It provides specifications for mullion systems with greater resistance to severe weather, than existed previously.
The AAMA 450 rating determines the air infiltration, water resistance, and structural performance of mulled fenestration assemblies. This higher standard has limited the number of suitable mullion systems that are available for multi-window constructions.
Additionally, anchored or fixed mullions are presently available in the form of “T” and “X” mullions. However, these anchored “T” and “X” mullions are formed of multiple, non-interchangeable parts and are difficult to assemble.
Furthermore, some conventional mullion systems suffer from rot and/or corrosion. For example, windows can lose their water tight seal from the time of manufacture until the time they are installed. This could be the result of stresses developed during shipping and handling, or simply improper handling and/or installation of the window system, as well as the improper assembly of the mullion itself.